Plasma sprayed titanium carbide tool steel coating

ABSTRACT

A composite article of manufacture is provided comprising a metal substrate having a plasma-deposited adherent coating of a titanium carbide tool steel containing by weight about 10 to 80 percent of primary grains of TiC and the balance essentially about 90 to 20 percent of a steel matrix, the matrix being characterized metallographically by an austenitic decomposition product containing martensite. The metal substrate is preferably aluminum, although other metal substrates, such as steel, can be coated. The method comprises quench-depositing the coating by plasma spraying onto the substrate such that the resulting coating is one characterized metallographically by the presence of martensite. Preferably, the titanium carbide grains should be rounded to assure a low friction surface.

United States Patent [191 Ellis et al.

[ PLASMA SPRAYED TITANIUM CARBIDE TOOL STEEL COATING [75] Inventors:John L. Ellis, Whiteplains; M.

Kumar Mal, Nanuet; Stuart E. Tarkan, Monsey, all of NY.

[73] Assignee: Chromalloy American Corporation, West Nyack, NY.

[22] Filed: Nov. 17, 1971 [21] Appl. No.: 199,497

[52] US. Cl. 29/195 A, ll7/93.l PF

[ Dec. 18, 1973 3,705,818 12/1972 Grosseau ll7/93.1 PF X PrimaryExaminer-L. Dewayne Rutledge Assistant Examiner-E. L. WeiseAttorney-Nichol M. Sandoe et a1.

[57] ABSTRACT A composite article of manufacture is provided comprisinga metal substrate having a plasmadeposited adherent coating of atitanium carbide tool steel containing by weight about 10 to 80 percentof primary grains of TiC and the balance essentially about 90 to 20percent of a steel matrix, the matrix being characterizedmetallographically by an austenitic decomposition product containingmartensite. The metal substrate is preferably aluminum, although othermetal substrates, such as steel, can be coated. The method comprisesquench-depositing the coating by plasma spraying onto the substrate suchthat the resulting coating is one characterized metallographically bythe presence of martensite. Preferably, the titanium carbide grainsshould be rounded to assure a low friction surface.

7 Claims, 2 Drawing Figures PLASMA SPRAYED TITANIUM CARBIDE TOOL STEELCOATING This invention relates to a method for producing an adherent,hard, wear resistant coating of a heat treatable titanium carbide toolsteel onto a metal substrate (e.g., steel) and, in particular, to amethod for producing such coatings on relatively soft metal substrates,such as aluminum, copper, silver and the like, whereby the hard titaniumcarbide steel coating can be further heat treated, if desired, attemperatures below the melting point of the substrate metal. Theinvention also relates to composite metal structures produced by themethod.

STATE OF THE ART It is known to hard face metal substrates by usingwelding and brazing methods in which the metal substrate issimultaneously heated during the laying down of the hard facingmaterial. Because of the general nature of the foregoing process, themetal substrates were limited to those having fairly high meltingpoints, otherwise, the substrate would overheat and either melt or beadversely affected.

One attempt to enlarge the use of hard facing has been to employ flamespraying. This method comprises melting powder metal compositions in aheated zone and propelling the molten particles to the surface of ametal substrate to form a coating thereon. This method had itslimitations as to the type of materials that could be sprayed. Forexample, if refractory carbide particles are sprayed, generally a matrixmetal powder is mixed with it, e.g., nickel, cobalt, etc., and themixture sprayed to provide the means by which to anchor the carbideparticles to the receiving surface. So long as no further heat treatmentis required of the coating, certain types of hard coatings could beproduced, although they tended to be porous.

In recent years, a special kind of hard titanium carbide tool steel hasbeen developed which, besides having the intrinsic hardness of thetitanium carbide, also is capable of being further hardened very much astool steel is hardened. For example, titanium carbide tool steelcontaining 33 percent by weight of TiC (about 45 percent by volume) andthe balance a chromiummolybdenum steel (note US. Pat. Nos. 2,828,202 and3,416,976) requires a relatively high temperature for heat treatment.Thus, to obtain a martensitic matrix, the titanium carbide tool steelcomposition is quenched from about l,750 F in oil. However, theforegoing heat treating temperature is higher than the melting point ofcertain metal substrates, such as aluminum. Moreover, the conventionallysprayed coatings tend to be quite porous.

Hard carbide coatings would be desirable on certain substrate metals,such as metals having relatively high thermal and electricalconductivity, for example, aluminum, copper, silver and the like. Itwould be desirable to provide such coatings having minimum porosity andexceptionally good wear resistance. Such coatings would be useful inproviding long life electric contact metal having a hard, wear resistantcontact face and a substrate of good thermal and electricalconductivity. Such coatings would also be useful in producing aluminumelements, e.g., a housing for the recently developed rotary combustionengine, having a hard surface coating to provide resistance to wearrelative to the rotary piston in contact therewith.

OBJECTS OF THE INVENTION Thus an object of the invention is to provideas a composite article of manufacture a metal substrate having anadherent dense coating of a titanium carbide tool steel comprisedmetallographically of primary grains of titanium carbide dispersedsubstantially uniformly through a steel matrix characterized by anaustenitic decomposition product comprising martensite. Other hardphases may be present, such as bainite, and the term martensite employedherein is meant to cover martensite with or without retained austeniteand mixtures of martensite with bainite, with or without retainedaustenite.

An additional object is to provide a titanium carbide tool steel coatingcharacterized metallographically by rounded primary grains of titaniumcarbide.

Another object is to provide a titanium carbide tool steel coating onmetal substrates of melting point above 1,l00 F.

A still further object of the invention is to provide a titanium carbidetool steel hard-facing coating on a metal substrate of aluminous metalin which the coating is comprised metallographically by primary grainsof titanium carbide dispersed substantially uniformly through a steelmatrix characterized by an austenitic decomposition product containingmartensite.

These and other objects will more clearly appear when taken intoconsideration with the following disclosure and the accompanyingdrawing, wherein:

FIG. 1 depicts schematically a device for the plasma flame spraying ofmetal powder; and

FIG. 2 shows schematically a rotary combustion engine utilizing a heattreatable, titanium carbide steel as a hardfacing material on the innerend walls thereof.

STATEMENT OF THE INVENTION Stating it broadly, the method aspect of theinvention for producing a wear resistant coating of a heat treatabletitanium carbide tool steel on a metal substrate resides in selecting apowder composition consisting essentially of about 10 to percent byweight of primary grains of titanium carbide and the balance essentiallyto 20 percent by weight of steel-forming ingredients andquench-depositing said composition from the molten state onto a metalsubstrate by means of a plasma flame which heats the steel ingredientsto substantially above the melting point, whereby a dense, adherentcoating of the composition is produced on the metal substrate, thecoating being thin relative to the metal substrate and preferablyranging up to about 0.025 inch in thickness.

By employing the plasma flame for depositing the coating, rather hightemperatures are obtained which melt the steel matrix of the compositionat temperatures substantially above the melting point, such that thincoatings deposited on the metal substrate are drastically quenched byvirtue of the cooling effect of the substrate to produce amicrostructure comprising grains of titanium carbide dispersed through amatrix formed of an austenitic decomposition product containingmartensite. The metal substrate should preferably have a melting pointabove l,100 F.

As stated hereinabove, very high temperature are obtainable with theplasma flame. However, for most spray applications, a plasma temperatureof about 12,000 to 20,000 F appears to be optimum. One of the advantagesof the plasma flame is that it can be used with a controlled atmosphere.This is important to avoid decarburization of the steel matrix wherecarbon is essential to the heat treatment response of the titaniumcarbide tool steel. Thus, an inert gas or a chemically inactive gas canbe employed for the flame medium.

The plasma flame is produced by striking an are between a cathode and ananode and passing a plasma gas through the arc. By confining the arc ina chamber under pressure, the arc temperature can be increased. Byconstructing the anode as a hollow nozzle and introducing the plasma gasinto the arc chamber and forcing it through the nozzle, the gasdissociates and ionizes in the arc stream, and emerges from the nozzleas a plasma flame. A typical plasma gas is one comprised of 90 percentnitrogen and 10 percent hydrogen. Argon or other gases can be used inplace of nitrogen.

A schematic representation of a plasma flame device is given in theaccompanying drawing which shows cathode l and anode lll electricallyconnected via power source 12 to produce an arc stream 13. Plasma gas14, e.g. 90 percent nitrogen and percent hydrogen, is fed through pipe15 which is converted to plasma 16 which exits through nozzle 17 at avery high temperature as free plasma. Spray powder is fed through pipe19 into the nozzle where it is heated by the plasma flame and exits withthe free plasma towards the workpiece or substrate to be coated. Plasmaguns for metal powder spraying are ready available and therefore neednot be discussed any further than the schematic described above.

One of the advantages of plasma spraying is that relatively thincoatings can be sprayed which are dense and substantially free of largepores. By spraying thicknesses ranging up to about 0.025 inch, highlyrapid quenching of the deposit is obtained generally comprised ofmartensite. Where the metal substrate is a metal of substantially highthermal conductivity, e.g., aluminum, copper, silver and the like, veryhard coatings are obtained which can be further heat treated, e.g.,tempered or aged, at temperatures below the melting point of thesubstrate. However, the coating can also be applied with advantageousresults to ferrous metal substrates e.g., steels.

Details of the Invention As illustrative of titanium carbide tool steelcompositions that can be plasma sprayed onto metal substrate, thefollowing examples are given.

EXAMPLE 1 Broadly, the titanium carbide tool steel is comprisedessentially of about 10 to 80 percent by weight of primary grains oftitanium carbide dispersed through a steel matrix making up about 80 to20 percent of the balance. The steel may be low and high carbon steel,medium alloy steel or high alloy steel containing at least 50 percentiron which, when cooled substantially rapidly from above the meltingpoint, provides metallographically a matrix containing an austeniticdecomposition product containing martensite. In this connection,reference is made to US. Pat. No. 2,828,202. Examples of such matrixsteels are: SAE 1010 to SAE 1080 steels, and including the followingillustrative composition, to wit: 0.8 percent Cr, 0.2 percent M0, 0.3percent C and iron substantially the balance; 5 percent Cr, 1.4 percentM0, 1.4 percent W, 0.45 percent V, 0.35 percent C and iron substantiallythe balance; 8 percent Mo, 4 percent Cr, 2 percent V, 0.8 percent C andiron substantially the balance; 18 percent W, 4 percent Cr, 1 percent V,0.75 percent C and iron substantially the balance; 20 percent W, 12percent Co, 4 percent Cr, 2 percent V, 0.8 percent C and ironsubstantially the balance.

A preferred composition is one containing about 1 to 6 percent Cr, about0.3 to 6 percent Mo, about 0.3 to 0.8 percent C and the balanceessentially iron.

Example 2 A particular titanium carbide tool steel is one containing 10to percent by weight of TiC and the balance essentially a high chromiumhigh carbon steel containing about 6 or 7 percent to 12 percentchromium, 0.6 to 1.2 percent carbon, 0.5 to 5 percent molybdenum, up toabout 5 percent tungsten, up to about 2 percent vanadium, up to about 3percent nickel, up to about 5 percent cobalt, and the balanceessentially iron. A preferred composition of the foregoing highchromium, high carbon steel is one containing about 10 percent chromium,1 percent carbon, 3 percent molybdenum, 1 percent vanadium, and thebalance essentially ironl This steel is characterized in that it formsmartensite when applied from a plasma spray onto a relatively coldsubstrate, e.g. steel, aluminum, and the like and, by double temperingat l,0O0 F for one hour each, the hardness is further augmented bysecondary hardening, while, at the same time, the coating issubstantially stress relieved of any thermal stresses due to the rapidcooling when deposited. As will be noted, the tempering temperature1,000 F) is below the melting point of aluminum.

Example 3 As illustrative of another titanium carbide tool steelcomposition that can be plasma sprayed onto a metal substrate and whichcan be further heat treated at a temperature below the melting point ofthe substrate metal is a heat treatable, low carbon nickel-containingtitanium carbide tool steel (note U.S. Pat. No. 3,369,891 As in theforegoing examples, the titanium carbide ranges from about 10 percent to80 percent by weight and the steel matrix from about percent to 20percent by weight. The matrix composition contains by weight about 10 to30 percent nickel, 0.2 to 9 percent of titanium, and up to about 5percent aluminum, the sum of the titanium and aluminum not exceedingabout 9 percent, up to about 25 percent cobalt, up to about 10 percentmolybdenum, and substantially the balance of the matrix at least about50 percent iron; the metals making up the matrix composition beingproportioned such that when the nickel content ranges from about 10percent to 22 percent and the sum of the aluminum and titanium is lessthan 1.5 percent, the cobalt and molybdenum contents are each at leastabout 2 percent; and such that when the nickel content ranges from about18 to 30 percent and the molybdenum content is less than 2 percent, thesum of the aluminum and titanium exceeds 1.5 percent.

When the foregoing titanium carbide tool steel is deposited from theplasma flame and rapidly quenched, the metallographic structure isessentially soft martensite. In this condition, the carbide steel in theform of a coating can be age-hardened by heating it to a temperature ofabout 500 to 1,200 F (260 to 650 C) for about 3 hours. A typicalage-hardening temperature is 900 F (483 C). As will be noted, theage-hardening temperature is below the melting point of aluminum.Normally, the solution temperature for obtaining soft martensite in thematrix ranges from about l,400 to 2,150 F (760 to 1,165 C). As will beobserved, the foregoing temperature range is above the melting point ofaluminum. However, a solution treatment is not necessary for the coatingsince the steel is solutionquenched due to the rapid cooling followingplasma spraying of the composition.

' typicafcoinposition is' one containing about 35 percent by weight ofTiC and the 65 percent remainder a steel matrix containing 21.7 percentNi, 8.49 percent Co, 3.42 percent Mo, 0.37 percent Ti and the balanceessentially iron. The alloy upon aging at 900 F (483 C) for three hoursexhibited a hardness of about 60 R It is preferred when working withcompositions of the type illustrated in Examples 1, 2 and 3 to work witha pre-alloyed titanium carbide tool steel. This assures the presence ofrounded grains of titanium carbide which not only provides resistance towear but also imparts very low coefficient of friction. This isimportant in applications involving the continuous rubbing of parts,such as occurs in the rotary combustion engine where the apices of therotary piston are in continuous contact with the inner side walls of thehousing. Where the housing is made of aluminum, a face coating of atitanium carbide tool steel of the type illustrated in Example 2provides adequate resistance to wear and in addition low coefficient offriction by virtue of the rounded grains of TiC.

In assuring the rounded grain structure of titanium carbide, thetitanium carbide tool steel is pre-alloyed by liquid phase sintering apowder metallurgical composition of the carbide steel. The pre-alloyedcarbide steel is then ground into a particle size passing through 200mesh for use in plasma spraying.

ln producing a pre-alloyed steel composition with rounded grains of TiC,the following method is employed.

A titanium carbide tool steel composition containing 33 percent byweight of TiC (45 percent by volume) and substantially the balance asteel matrix, such as a chromium-molybdenum steel composition, isproduced by mixing 500 grams TiC (of about 5 to 7 microns in size) with1,000 grams of steel-forming ingredients in a mill half filled withstainless steel balls. To the powder mix is added 1 gram of paraffin waxfor 100 grams of mix. The milling is conducted for about 40 hours usinghexane as a vehicle. A specific steel-forming composition for the matrixis one containing 0.5 percent C, about 3 percent Cr, about 3 percent Moand the remainder substantially iron. it is preferred to use carbonyliron powder in producing the mixture. A carbidic tool steel of theforegoing type is disclosed in US. Pat. No. 3,416,976.

Following completion of the milling, the mix is removed and dried andcompacts of the desired shape pressed at about t.s.i. and the compactsthen subjected to liquid phase sintering in vacuum at a temperature ofabout 2,640 F (1,450 C) for about one-half hour at a vacuumcorresponding to microns or less. After completion of the sintering, thecompacts are cooled and then removed from the furnace. The primarytitanium carbide grains which are angular before sintering, assume arounded configuration as a result of liquid phase sintering. By liquidphase sintering is meant heating the compact to above the melting pointof the steel matrix but below the melting point for titanium carbide,for example, up to about 180 F C) above the melting point of the steelmatrix.

Following the production of the sintered compact, the sintered compactmay be converted into chips by machining and the chips milled in a ballmill to a size passing through 200 mesh (e.g., l to 5 microns). Thepowder is cleaned and dried for use for plasma flame spraying. As statedabove, rounded titanium carbide grains are preferred in the ultimatecoating since this configuration imparts low friction characteristics tothe coating, the rounded grains being advantageous in wear application.

Wear is a combination of corrosion, erosion, abrasion, friction,sulfidation, fatigue, fretting and oxidation in which the net result issurface deterioration. An advantage of using a titanium carbide steel asa hardfacing material is that it has a low density compared to otherhard-facing materials and provides good resistance to the foregoingphenomena. It has certain economic advantages since a unit weight of theforegoing hard-facing material on a volume basis covers more surfacearea of a metal substrate in comparison to the conventional hard-facingmaterials containing tungsten carbide which have a substantially higherdensity.

The rounded grain structure of titanium carbide, as stated hereinabove,is ideal because it imparts a low coefficient of friction to the coatingand also because titanium carbide has a very high intrinsic hardnessand, therefore, exhibits a very high resistance to wear. Moreover, theas-coated surfaces of this hard-facing material obtained after plasmaspraying are quite smooth, e.g., about 100 to microinches rms, otherwise referred to as root-mean square average" (square root of meansquare). This is advantageous because the coated surface can beinexpensively buffed to provide maximum resistance to wear, resistanceto galling and low friction. The dense deposited coating can be finishedto a smoothness of less than 5 microinches rms, using diamond lappingcompounds and other specialized techniques.

in plasma spraying a metal substrate, the surface thereof is degreased,cleaned and preferably gritblasted with chilled iron grit or purealuminum oxide grit to insure good adhesion of the hard-facing alloy tothe base material. The hard-facing material in the finely powdered form(200+325 mesh) is fed into the stream of a superheated plasma gas. Theparticles are melted and are carried by the gas at high velocity to thesurface being plated. A coating can be built up to the desired thicknessby forming multiple layers. It is preferred that the coating be thin andpreferably range up to about 0.025 inch and, more preferably, up toabout 0.015 inch, to minimize cracking due to cooling stresses. Comparedto most conventional coatings, the titanium carbide steel coatingassures good mating compatability with metal substrates due to its lowcoefficient of thermal expansion. This property makes this materialattractive for wear resistant applications in the automotive andaircraft industries.

As illustrative of a preferred embodiment of the invention, thefollowing example is given:

Example 4 A pre-alloyed titanium carbide tool steel produced by liquidphase sintering was employed as the plasma spray powder, the mesh sizeof the powder being in the range of about l70 to +325, the compositionconsisting essentially of about 33 percent by weight of TiC and 67percent by weight of a steel matrix having the following composition byweight: 3 percent Cr, 3 percent M0, 0.5 percent C and the balanceessentially iron.

A plate of aluminum (AMS-4026) was employed as the metal substrate. Teeplasma gas used comprised 90 percent nitrogen and percent hydrogen. Thealuminum surface was degreased and grit blasted with pure aluminum oxide(60 mesh) grit to provide a surface 7 to promote adherence of thecoating. Two coatings of 0.007 inch were produced, one by spraying inair and the other by plasma spraying in air using an argon shield tominimize formation of oxides in the coating.

The plasma spraying was conducted using a plasma spray gun referred toin the trade as Metco plasma flame spray system consisting of aspecially constructed torch-type gun in which powdered coating material,suspended in a suitable carrier gas (N was fed into a chamber in whichplasma gas was excited to high temperatures by an electric arc.

Metallographic examination of each of the coatings showed theconstituents of the coating to be uniformly distributed. The coatingswere substantially free from cracks, massive porosity characteristic ofconventionally flame sprayed coatings and substantially free fromexcessive oxides. The specimen sprayed with the argon shield showed lessoxides present than the one sprayed without the shield. The coatingswere essentially free from inclusions at the coating substrate metalinterface. The interface itself appeared to be well pronounced withpractically no porosity. The overall porosity in the coating wasapproximately 8 or 9 percent which isconsidered good.

Microhardness of the thin coating was determined across its thicknessand found to range from about 650 to 770 VHN (200 gram load) whichcorresponds to a Rcokwell C hardness of about 52 to 60. The hardness ischaracteristic of the presence of an austenitic decomposition productcontaining martensite. The foregoing steel composition in the-annealedstate (pearlitic microstructure or spheroidized carbon) normally has ahardness in the neighborhood of 40 R Thus, the invention enables theproduction on an aluminous metal substrate with a hardened titanium carbide steel coating without the necessity of quench hardening the coatingfrom an austenitizing temperature of about l,750 F which is above themelting point of the aluminous substrate. The quenching effect achievedduring the deposition of the coating provides the desirable austeniticdecomposition product.

No spalling, chipping, flaking, cracking, and the like, were observed onthe coating surface and the general quality was good.

In a test to evaluate the adherence of the coating, a test panel ofaluminum (AMS-4026) of about 3 l.75 and 0.05 inches in size was plasmacoated with the same steel composition to a thickness of 0.005 inch. Thecoated panel was then subjected to a cup test in accordance with thePratt & Whitney Aircraft Materials Control Laboratory Manual (SectionE-53, 1963 Revision) using a 0.875 inch diameter ball and a die with a1.375 inch diameter opening to form a depression in the panel ofapproximately 0.300 inch. The coating did not exhibit any separationfrom the base metal indicating that the plasma sprayed coating had goodbond strength.

A quench deposited coating of the same steel composition was produced onaluminum having a thickness of about 0.0l 5 inch. This coating had aquench-deposited hardness of over 50 R and up to about 60 R indicatingthe presence of martensite in the coating. The coating was uniform,dense and crack-free. Hardened coatings of this type can be tempered attemperatures below the melting point of the metal substrate. A typicaltemperature for this steel may range from about 200 to 500 F.

Example 5 A hard-facing composition particularly resistant to softeningat elevated temperatures ranging up to about 1,000 F is one comprisingabout 35 percent by weight of TiC and the balance percent of a steelmatrix comprising essentially about 10 percent Cr, 3 percent M0, 0.8percent C and the balance essentially iron. As in Example 4, this steelcomposition is provided essentially in the pre-alloyed condition toassure the presence of rounded primary grains of TiC dispersed throughthe steel matrix.

A powder of the foregoing titanium carbide tool steel of 200 +325 meshis plasma sprayed onto a mild steel substrate of about one-fourth inchthick to produce a coating thickness of about 0.01 inch, the producedcoating being quench deposited by virtue of the mass of metal substratewhich rapidly cools the coating sufficiently to provide amartensitic-bearing metallographic structure. The hardness of thiscoating will generally be in the range of about 50 to 55 Rockwell C.However, this steel composition can be further hardened by utilizing itssecondary hardening characteristic by the formation of secondarycarbides by heating the coated metal substrate and the coating to aboutl,000 F and holding at the temperature for about 1% hours. Thus, heatingto l,000 F will have a two-fold function; (1) to utilize the secondaryhardening characteristics of the titanium carbide steel composition, and(2) to minimize the effect of any residual thermal stresses in thecoating arising from the rapid quenching of the coating duringdeposition from the plasma flame.

Example 6 In addition to the foregoing example, some heat treatment datawere obtained on two plasma-coated aluminum substrate specimens.

Substrate A was coated with a pre-alloyed titanium carbide tool steelcontaining 35 percent by weight of TiC and the balance a steel matrixcontaining 3 percent Cr, 3 percent M0, 0.5 percent C with the remainderiron.

Substrate B was coated with a pre-alloyed titanium carbide tool steelcontaining 35 percent by weight of TiC and the balance 10 percent Cr, 3percent M0, 0.8 percent C with the remainder iron.

Hardness readings were obtained for the coatings as sprayed and for thecoatings double tempered at 975/950 F. Each was tempered for one hour attemperature, cooled and tempered again for one hour. The resultsobtained are given in the following table.

Aluminum Microhardness Rockwell C Sub- Condition (Vickers) (Converted)strate A As sprayed 859 (Av. of 7 readings) 65.9 Double Tempered 788(Av. of 7 readings) 63.5 B As sprayed 620 (Av. of readings) 56.5 DoubleTempered 1048 (Av. of 6 readings) 69.4

It will be noted that the hardness on Substrate A only dropped a fewpoints after double tempering; whereas, the hardness on Substrate Bincreased markedly by about 13 points, apparently due to secondaryhardening.

Example 7 This hard-facing titanium carbide steel composition isadvantageous in that the coating deposited on a substrate, such asaluminum, can be hardened after deposition by heat treatment at atemperature below the melting point of aluminum or other substratemetal. A particular composition is one containing 30 percent by weightof titanium carbide and the balance of 70 percent essentially a steelmatrix containing about 21.5 percent Ni, 8.5 percent Co, 3.4 percent M0,0.4 percent Ti and the balance essentially iron.

The foregoing steel in the pre-alloyed condition and as a powder ofl50+325 mesh size is plasma sprayed onto aluminum as described in Example4. By using a steel matrix very low in carbon, e.g., below 0.15 percent,soft martensite is obtained in the coating. After the substrate has beencoated, the substrate and the coating are heated to a temperature ofabout 900 F as described in Example 3 to age harden to steel to thedesired hardness.

An advantage of the foregoing steel composition is that the coating canbe buffed to the desired smoothness and then age hardened at theforegoing temperature to desired hardness.

The methods described in Examples 4 to 7 can be applied to a largevariety of metal substrates. The method is particularly applicable tothe coating of metal substrates of relatively high thermal andelectrical conductivities, such as aluminum, copper, silver and thelike. In this connection, the invention is applicable to such metalsubstrates as steels, iron-base alloys, nickel and nickel-base alloys,cobalt and cobalt-base alloys and, generally to those metals havingmelting points above 1,200 F.

The invention is particularly applicable to those metal substrateshaving thermal and electrical conductivities of at least 0.2 referred tohigh electrical and thermal conductivity copper taken as 1. Thus, theinvention is applicable to the coating of the metals aluminum, copperand silver and Al-base, Cu-base and Agbase alloys. The metals copper andsilver and their alloys have particular use in electrical contacts wherewear resistance of the contacting face may be important.

Certain steel substrates can be further augmented in their properties byplasma spraying thereon a titanium carbide tool steel composition of thetype disclosed herein. One advantage of plasma spraying such substratesis in the building up of worn dies or dies' inadver tently produced withan undersize.

A particular hard facing coating mentioned hereinbefore is onecontaining percent to 80 percent by weight of TiC and the balance asteel matrix containing about 6 or 7 percent to 12 percent Cr(preferably 8 to 12 percent), about 0.6 to 1.2 percent C, about 0.5 to 5percent Mo, up to about 5 percent W, up to about 2 percent V, up toabout 3 percent Ni, up to about 5 percent Co and the remainderessentially iron.

A specific coating composition is one containing about 40 to 50 percentby weight TiC, and the balance a steel matrix containing about 10percent Cr, about 1 percent C, about 3 percent Mo, about 1 percent V andthe remainder essentially iron. Examples of two steel substrates coatedwith the foregoing type of composition are as follows.

Example 8 The coating is applicable to a steel substrate bearing theAIS] designation S2 (silicon tool steel) having the followingcomposition: 0.5 0.6 percent C, 0.4 0.6 percent Mn, 0.7 1.2 percent Si,0.15 0.3 V, 0.4 0.6 percent Mo and the balance essentially iron. Thissteel exhibits good resistance to shock. This property can be furtheraugmented by providing a hard coating on the surface thereof by plasmaspraying the following composition by weight: about 40 percent TiCdispersed through a steel matrix containing about 10 percent Cr, 1percent C, 3 percent Mo, 1 percent V and the re mainder essentiallyiron. The coating is deposited (about 0.0l5 inch thick) by plasmaspraying in accordance with the method of Example 4. After the coatinghas been smoothed by buffing, it is tempered at about 950/975 F forabout 1 hour, cooled and then tempered again at the same temperature forone hour to increase the hardness of the coating still further.Following tempering, the coated substrate is ground to finished size.Generally, the tempering temperature may range from about 900 to l,050F.

Example 9 A worn die punch of a titanium carbide tool steel is built upby plasma spraying the surface thereof with a coating compositionsimilar to Example 8 containing by weight about 50 percent TiC dispersedthrough a steel matrix containing 10 percent Cr, 1 percent C, 3 percentMo and the balance essentially iron. The composition of the substratecomprises 35 percent TiC by weight in a steel matrix containing 3percent Cr, 3 percent Mo, 0.5 percent C and the balance essentiallyiron. The coating is deposited to a thickness of about 0.01 inch,smoothed by bufi'mg and then double tempered as in Example 8 at 950l975F. Following tempering, the coated die punch is finished ground to thedesired dimension. While the substrate is hard, the coating providesadditional hardness and markedly improved wear resistance in light ofthe higher titanium carbide content.

Thus, the invention provides as articles of manufacuture composite metalstructures of ferrous and nonferrous metal substrates with adherentcoatings of titanium carbide steel compositions. Examples of suchcomposite metal structures produced by plasma spraying are as follows.

- Aluminum 35% TiC and 65% steel matrix containing 5% Cr, L496 M0, 1.4%W, 0.45% V, 0.35% C and the balance essentially iron Aluminum 40% TiCand 60% steel matrix containing 8% Cr, 3% Mo, l% V, 0.9% C and thebalance essentially iron Copper 35% TiC and 65% steel matrix containing20% Ni, 1.75% Ti, 0.8% A1, 0.l5% C, 0.5% Mn, 02% Si and the balanceessentially iron Silver 60% TiC and 40% steel matrix containing l% Cr,2% Mo, 2% W, 1% C and the balance essentially iron As will be apparent,the metal substrate may be selected from the group consisting of steeland nonferrous metals. The non-ferrous metals of particular use assubstrates are aluminum, copper and silver and alloys based on thesemetals, e.g., aluminum-base, copper-base and silver-base alloys.

As stated hereinbefore, the coating material should preferably bepre-alloyed before spraying so as to assure the presence of roundedgrains of primary titanium carbide dispersed through amartensiticcontaining steel matrix. The coatings in the foregoing tableby virtue of plasma spraying are generally characterized by the presenceof martensite in the matrix. This assures a hard matrix surrounding thetitanium carbide grains. If the matrix is too soft, then preferentiallywearing of the matrix will result in dislodgement of titanium carbideand the gradual fretting away of the coating material.

Present developments in the rotary combustion engine contemplate the useof an aluminum housing. The rotating piston which has a generallytriangular shape is in contact with the end walls of the housing bymeans of the apices thereof which require the use of a seal material asa seal-off between the spaces defined between the apices. The seal musthave wear resistance. However, the aluminum in the housing is generallysoft compared to most materials of construction and has poor wearresistance. By plasma spraying the inner walls of the aluminum housingwith a hard-facing material of a titanium carbide tool steel compositionhaving low friction properties, the life of the housing can beprolonged. An aluminum housing is desirable as it is capable of beingair cooled easily in view of its high thermal conductivity. The heattreatable hard-facing material should preferably be temper resistant soas to resist softening due to the generation of heat in the pistonduring fuel combustion.

A coating material which appears promising in that regard for thealuminum housing is one containing by weight about 10 to 80 percent TiCand the balance 90 to 20 percent of a steel matrix containing 6 to 7 to12 percent Cr, 0.5 to percent M0, 0.6 to 1.2 percent C and the balancesubstantially iron. The matrix composition may contain up to about 5percent W, up to about 3 percent Ni, up to about 5 percent Co, up toabout 2 percent V, amounts of Mn and Si usually found in steel and thebalance essentially iron. A specific composition is one containingpercent Cr, 3 percent Mo, 1 percent C and the balance essentially iron.This steel resists tempering at temperatures as high as 1,000 F and infact increases in hardness due to a secondary hardening effect at thelatter temperature.

FIG. 2 shows schematically a rotary combustion engine comprising analuminum housing 20 having a chamber 21 in which is mounted atriangularly shaped rotary piston 22 in sealing contact with the endwall 23 of the chamber at its apices 24 to 26. The rotary piston has aninternal gear mounted thereon which is driven by gear 28 mounted on ashaft running perpendicular to the rotary piston. The hard-facingmaterial is applied to end wall 23 as shown by the heavy line to providesufficient wear resistance to the material of the apices in rubbingcontact with the end wall. The material of the apices may comprisespring mounted inserts 29 of the same titanium carbide tool steelmaintained in continual sealing contact with the end wall via spring 30.

In operation, as the piston rotates, fuel and air are received at intakezone 31 through intake 32. The fuel-air mixture is then compressed andfired in compression zone 33 via spark plug 34 and the combusted gasesat exhaust zone 35 exhausted through outlet 36. The temperature in thechamber rises to levels which may tend to temper certain heat treatablesteels. However, by employing a hard-facing material describedhereinabove containing about 35 percent by weight of TiC dispersedthrough a steel matrix containing about 10 percent Cr, 3 percent Mo, 1percent C and the balance essentially iron, a temper resistant surfaceis provided capable of being heated up to about l,000 F withoutsubstantially diminishing in hardness and, if anything, increases inhardness due to a secondary hardening effect engendered by theprecipitation of secondary carbides containing chromium and/or throughdecomposition of retained austenite. By assuring the presence of roundedgrains of titanium carbide in both the inserts of the seal and the endwall hard-facing material, low friction is assured during operation ofthe engine.

It is to be understood that the term steel substrate referred to hereinalso includes cast steels, cast irons and the iron-base materials.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What is claimed is:

1. As a composite article of manufacture, a metal substrate having athin plasma-deposited adherent coating of a titanium carbide tool steelcontaining by weight about 10 to percent of rounded primary grains ofTiC dispersed through a steel matrix making up essentially the balanceof about to 20 percent by weight, the matrix of the coating in the asplasmadeposited state being characterized metallographically by anaustenitic decomposition product containing martensite.

2. As a composite article of manufacture, a metal substrate having athin plasma-deposited adherent coating of a titanium carbide tool steelcontaining by weight about 10 to 80 percent rounded primary grains ofTiC dispersed through a steel matrix making up essentially the balancein an amount ranging from about 90 to 20 percent by weight, said steelmartix being selected from the group consisting of (A) about 1 to 6percent Cr, about 0.3 to 6 percent Mo, about 0.3 to 0.8 percent C andthe balance essentially iron; (B) about 6 to 12 percent Cr, about 0.5 to5 percent Mo, about 0.6 to 1.2 percent C, up to about 5 percent W, up toabout 2 percent V, up to about 3 percent Ni, up to about 5 percent Coand the balance essentially iron; and (C) a high nickel alloy steelcontaining about 10 to 30 percent Ni, about 0.2 to 9 percent Ti, up toabout percent Al, the sum of the Ti and Al content not exceeding about 9percent, up to about 25 percent Co, up to about 10 percent Mo,substantially the balance of the matrix being at least about 50 percentiron, the metals making up the matrix composition being proportionedsuch that when the nickel content ranges from about 10 to 22 percent andthe sum Al and Ti is less than about 1.5 percent, the molybdenum andcobalt contents are each at least about 2 percent, and such that whenthe nickel content ranges from about 18 to 30 percent and the molybdenumcontent is less than 2 percent, the sum of Al and Ti exceeds 1.5percent; said matrix of said coating produced from compositions (A), (B)and (C) being characterized metallographically in the asplasma'dep'o'sited state by'the presence of martensite.

3. The composite article of claim 2, wherein the metal substrate isselected from the group consisting of steel and non-ferrous metalshaving a melting point about l,100 F and wherein the thin coating rangesin thickness up to about 0.025 inch.

4. The composite article of manufacture of claim 3, wherein the metalsubstrate is characterized by a thermal and electrical conductivityrelative to copper taken as l of at least about 0.2.

5. The composite article of manufacture of claim 4, wherein the metalsubstrate is selected from the group consisting of aluminum, copper andsilver and alloys based on these metals.

6. As a composite article of manufacture, an aluminous substrate havinga thin plasma-deposited adherent coating of a titanium carbide toolsteel containing by weight about 10 to 80 percent of primary grains ofTiC dispersed through a steel matrix making up essentially the balancein an amount ranging from about to 20 percent, the matrix of saidcoating being characterized metallographically in the asplasma-deposited state by an austenitic decomposition product containingmartensite.

7. The composite article of manufacture of claim 6, wherein the steelmatrix is selected from the group consisting of (A) about 1 to 6 percentCr, about 0.3 to 6 percent Mo, about 0.3 to 0.8 percent C, and thebalance essentially iron; (B) about 6 to 12 percent Cr, about 0.5 to 5percent Mo, about 0.6 to 1.2 percent C, up to about 5 percent W, up toabout 5 percent Co and the balance essentially iron; and (C) a highnickel alloy steel containing about 10 to 30 percent Ni, about 0.2 to 9percent Ti, up to about 5 percent Al, the sum of the Ti and Al contentnot exceeding about 9 percent, up to about 25 percent Co, up to about 10percent Mo, substantially the balance of the matrix being at least about50 percent iron, the metals making up the matrix composition beingproportioned such that when the nickel content ranges from about 10 to22 percent and the sum of Al and Ti is less than about 1.5 percent, themolybdenum and cobalt contents are each at least about 2 percent, andsuch that when the nickel content ranges from about 18 to 30 percent andthe molybdenum content is less than 2 percent, the sum of Al and Tiexceeds 1.5 percent; said matrix of said coating produced fromcompositions (A), (B) and (C) being characterized metallographically inthe as plasma-deposited state by the presence of martensite, and whereinsaid thin coating ranges in thickness up to about 0.025 inch.

' UNETED STATES PATENT OFFICE-- CERTEFICATE OF CORRECTION December 18,1975 Patent No. 577972O Dated Invent0r(s John Le Ellis et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Claim 5, column 13, line 20, "about llOOF" should read above 1100FSigned and sealed this 21st day of January 1975,

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Commissioner of Patents AttestingOfficer USCOMM-DC 60376-P69 U S GOVERNMENY PRINT NG OFFICE: 869-930 FORM PO-1050 (0-69) UNITED STATES PATENT OFFICE- CERTIFICATE OFCORRECTION December 18, 1975 Patent No 779 720 Dated Inventor(s JOhIl L.Ellis et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Claim 5 column 15, line 20, "about 1lOO F' should read above 'llOOFSigned and sealed this 21st day of January 1975.,

' (SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Commissioner of Patents Attesting;Officer USCOMM-DC 60376-P69 uvs covc ynnem PRINTING OFFICE: a 69 93 o FORM PO-IOSO (O-69)

2. As a composite article of manufacture, a metal substrate having athin plasma-deposited adherent coating of a titanium carbide tool steelcontaining by weight about 10 to 80 percent rounded primary grains ofTiC dispersed through a steel matrix making up essentially the balancein an amount ranging from about 90 to 20 percent by weight, said steelmartix being selected from the group coNsisting of (A) about 1 to 6percent Cr, about 0.3 to 6 percent Mo, about 0.3 to 0.8 percent C andthe balance essentially iron; (B) about 6 to 12 percent Cr, about 0.5 to5 percent Mo, about 0.6 to 1.2 percent C, up to about 5 percent W, up toabout 2 percent V, up to about 3 percent Ni, up to about 5 percent Coand the balance essentially iron; and (C) a high nickel alloy steelcontaining about 10 to 30 percent Ni, about 0.2 to 9 percent Ti, up toabout 5 percent Al, the sum of the Ti and Al content not exceeding about9 percent, up to about 25 percent Co, up to about 10 percent Mo,substantially the balance of the matrix being at least about 50 percentiron, the metals making up the matrix composition being proportionedsuch that when the nickel content ranges from about 10 to 22 percent andthe sum Al and Ti is less than about 1.5 percent, the molybdenum andcobalt contents are each at least about 2 percent, and such that whenthe nickel content ranges from about 18 to 30 percent and the molybdenumcontent is less than 2 percent, the sum of Al and Ti exceeds 1.5percent; said matrix of said coating produced from compositions (A), (B)and (C) being characterized metallographically in the asplasma-deposited state by the presence of martensite.
 3. The compositearticle of claim 2, wherein the metal substrate is selected from thegroup consisting of steel and non-ferrous metals having a melting pointabout 1,100* F and wherein the thin coating ranges in thickness up toabout 0.025 inch.
 4. The composite article of manufacture of claim 3,wherein the metal substrate is characterized by a thermal and electricalconductivity relative to copper taken as 1 of at least about 0.2.
 5. Thecomposite article of manufacture of claim 4, wherein the metal substrateis selected from the group consisting of aluminum, copper and silver andalloys based on these metals.
 6. As a composite article of manufacture,an aluminous substrate having a thin plasma-deposited adherent coatingof a titanium carbide tool steel containing by weight about 10 to 80percent of primary grains of TiC dispersed through a steel matrix makingup essentially the balance in an amount ranging from about 90 to 20percent, the matrix of said coating being characterizedmetallographically in the as plasma-deposited state by an austeniticdecomposition product containing martensite.
 7. The composite article ofmanufacture of claim 6, wherein the steel matrix is selected from thegroup consisting of (A) about 1 to 6 percent Cr, about 0.3 to 6 percentMo, about 0.3 to 0.8 percent C, and the balance essentially iron; (B)about 6 to 12 percent Cr, about 0.5 to 5 percent Mo, about 0.6 to 1.2percent C, up to about 5 percent W, up to about 5 percent Co and thebalance essentially iron; and (C) a high nickel alloy steel containingabout 10 to 30 percent Ni, about 0.2 to 9 percent Ti, up to about 5percent Al, the sum of the Ti and Al content not exceeding about 9percent, up to about 25 percent Co, up to about 10 percent Mo,substantially the balance of the matrix being at least about 50 percentiron, the metals making up the matrix composition being proportionedsuch that when the nickel content ranges from about 10 to 22 percent andthe sum of Al and Ti is less than about 1.5 percent, the molybdenum andcobalt contents are each at least about 2 percent, and such that whenthe nickel content ranges from about 18 to 30 percent and the molybdenumcontent is less than 2 percent, the sum of Al and Ti exceeds 1.5percent; said matrix of said coating produced from compositions (A), (B)and (C) being characterized metallographically in the asplasma-deposited state by the presence of martensite, and wherein saidthin coating ranges in thickness up to about 0.025 inch.